Increasing numbers of patients are being identified as suffering from disorders resulting from deficits in the mitochondrial respiratory chain and oxidative phosphorylation. These mitochondrial diseases display great diversity in clinical signs and symptoms and morphological and biochemical characteristics. While the identification of pathogenic mutations in nuclear and mitochondrial DNA has aided the genetic counseling of patients, the prognosis of patients is not good. Currently, there are no reliable treatments or therapies available for respiratory chain deficiencies. The specific aims of this grant application are to develop assay protocols suitable for high throughput screening to detect improvements in mitochondrial respiratory chain function. Transmitochondrial cell models that have been developed to study the pathogenic consequences of mitochondrial DNA mutations will form the basis of the assay systems. The primary screen will use known differences in growth properties of cells harboring mitochondrial DNA mutations to identify compounds that improve mitochondrial respiratory chain function, which subsequently result in improved growth characteristics. This assay will be optimized for automated high thoughput screening. Secondary assays and counter- screening assays for evaluating active compounds identified in the primary screen will also be developed and optimized. Identification of pharmacological probes that improve mitochondrial respiratory chain function in mutant cells will advance our knowledge of mitochondrial biogenesis and the mechanisms by which mitochondrial DNA mutations result in mitochondrial respiratory chain dysfunction. These compounds will facilitate the development of therapies for patients with mitochondrial diseases. PUBLIC HEALTH RELEVANCE: Mitochondria are important for the function of the cells in our body. Mutations in the genetic material of mitochondria result in a variety of human diseases. The studies proposed here will help define the mechanism by which these mutations result in mitochondrial dysfunction and identify potential therapies for human diseases resulting from mitochondrial DNA mutations.